Microglial Acid Sensing Regulates Carbon Dioxide-Evoked Fear

Abstract

Background Carbon dioxide (CO2) inhalation, a biological challenge and pathological marker in Panic Disorder, evokes intense fear and panic attacks in susceptible individuals. The molecular identity and anatomical location of CO2-sensing systems that translate CO2-evoked fear remains unclear. We investigated contributions of microglial acid sensor T cell death associated gene-8 (TDAG8) and microglial pro-inflammatory responses in CO2-evoked behavioral and physiological responses. Methods CO2-evoked freezing, autonomic and respiratory responses were assessed in TDAG8-deficient (−/−) and wildtype (+/+) mice. Involvement of TDAG8-dependent microglial activation and pro-inflammatory cytokine IL-1β with CO2-evoked responses was investigated using microglial blocker, minocycline and IL-1β antagonist, IL- 1RA. CO2-chemosensitive firing responses using single-cell patch clamping were measured in TDAG8−/− and +/+ mice to gain functional insights. Results; TDAG8 expression was localized in microglia enriched within the sensory circumventricular organs (CVOs). TDAG8−/− mice displayed attenuated CO2-evoked freezing and sympathetic responses. TDAG8 deficiency was associated with reduced microglial activation and pro-inflammatory cytokine, IL-1β within the subfornical organ (SFO). Central infusion of microglial activation blocker, minocycline and IL-1β antagonist, IL-1RA attenuated CO2-evoked freezing. Finally, CO2-evoked neuronal firing in patch clamped SFO neurons was dependent on acid sensor TDAG8 and IL-1β. Conclusions Our data identify TDAG8-dependent microglial acid-sensing as a unique chemosensor for detecting and translating hypercapnia to fear-associated behavioral and physiological responses, providing a novel mechanism for homeostatic threat detection of relevance to psychiatric conditions such as panic disorder.

Differential behavioral sensitivity to carbon dioxide (CO2) inhalation in rats

“…Previous reports of CO 2 inhalation in rats have reported freezing behavior using a similar concentration (Mongeluzi et al, 2003). 10% CO 2 also evokes freezing in mice as reported by our group (Vollmer et al, 2016), and others (Ziemann et al, 2009; Taugher et al, 2014, Liebold et al, 2016). Thus CO 2 -evoked immobility/freezing behavior appears to be consistent across species and strains and likely represents a fear-associated defensive behavior.…”

“…Additionally, orexin neurons in the lateral hypothalamic area are CO 2 -senstive and are relevant to the behavioral and physiological effects of CO 2 inhalation (Johnson et al, 2012). In recent studies, our lab reported regulation of CO 2 -evoked fear by microglial acid sensing mechanisms (Vollmer et al, 2016). It will be important to tease out contributions of these targets to differential CO 2 sensitivity.…”

“…Animals were exposed to a three day paradigm (see Fig 1) consisting of habituation (day 1), air or CO 2 exposure (day 2) and CO 2 -context exposure (day 3) in the absence of CO 2 as described in previous studies from our group (Vollmer et al, 2016), with modifications. This enabled an assessment of unconditioned (day 2) and conditioned (day 3) behavioral responses to CO 2 inhalation.…”

Adolescent food restriction in rats alters prefrontal cortex microglia in an experience-dependent manner

“…A second analysis of the same regions was performed to determine morphological differences in microglia, as previously described (Vollmer et al, 2016). Briefly, acquired images were analyzed using ImageJ software to quantify soma area and microglial process complexity and length (ramification).…”

Modern Microglia: Novel Targets in Psychiatric Neuroscience

“…They also express receptors that allow them to sample the extracellular milieu of the brain. For example, in this issue of Biological Psychiatry , research by Vollmer et al suggests that microglia detect the acidotic conditions triggered by carbon dioxide exposure through the acid-sensing receptor T-cell death–associated gene-8 (4). Microglia are also sensitive to neuronal activity that is driven by sensory stimuli transmitted from the external world.…”